Hyperspectral imaging

The first stage in the process is to crush the objects we want to sort. Nothing very technical up until this point. In the more rigorous second stage, this waste is scrutinised by the camera to spot “what is what”. The camera used for this purpose is a hyperspectral camera. This is a traditional black and white or colour camera with a prism and/or a network added after the lens, allowing the light to be separated into its various components.

Therefore, while an ordinary colour camera “sees” three colours (red, green and blue), a hyperspectral camera can distinguish up to 512 colours, and even more beyond that which is visible, from ultraviolet to infrared. Each colour is then associated with its wavelength, which allows a curve to be drawn composed of the various wavelengths of the light reflected by the material. A precise curve therefore corresponds to a certain material, which constitutes its “ID card”.

Liège’s contribution

Optics is one of CSL’s strengths and although the centre in Liège masters hyperspectral imaging, CSL’s contribution to the programme concerns the lighting of elements to be sorted and the automation of the system. While the latter only requires traditional industrial techniques, it is another matter for lighting. In order to optimise the perception of the material filmed, CSL was given the responsibility of developing the purest and most uniform beam of light possible. Because variations in light intensity are expressed by variations in colour (we all know about this phenomenon with halogens, for instance), the luminous flux must be as constant as possible. Of course, the luminous flux must also be even along the entire conveyor system. The flux tested by CSL is 10 millimetres wide and 650 millimetres long. It is obtained thanks to the reflection of a light in a double concave space.

When the materials have been picked up by the camera, the final operation is the actual sorting. The system notes the spatial coordinates of the elements and activates small pneumatic ejectors, which tips them into the appropriate containers.